Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y : list B
  ============================
   ({| pr1 := [x]; pr2 := y |} = {| pr1 := []; pr2 := [] |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
simplify * || simplify ?